Fixed precision in progress percentage display (#512)

Currently, when loading a file for processing, the percentage display
underneath the progress bar jumps around with arbitrary float precision.
Fixing it to 2 decimal places would make it more readable.

src/loader.ts

@@ -474,7 +474,7 @@ class Loader extends LitElement {
       ${displayPercent
         ? html`
             <label class="progress-label" for="progress"
-              >${displayPercent}%</label
+              >${displayPercent.toFixed(2)}%</label
             >
           `
         : nothing}